This invention relates to conducting laser energy from a laser energy source along a course that includes curves of small radius.
In many circumstances in various industrial and medical applications, matter to be cut or welded or otherwise altered or removed is located at a site that is inaccessible or difficult to reach.
Many sites within the body of an animal such as a human patient are difficult to reach for performing surgery, because they are surrounded by hard tissues such as bone or because they are surrounded by delicate tissues which can be damaged. Sites within the thorax, such as the heart and the blood vessels near it, for example, are enclosed by bone structures, and sites within the cranium, such as arteries supplying the brain, for example, are surrounded by delicate brain tissue as well as by bone. The coronary arteries and the arteries of the brain can become occluded for example by atheromatous plaque formations or by thrombi or emboli, with serious consonances for the patient.
One approach to providing a supply of blood to the heart when a coronary artery is occluded is bypass surgery, that is, coronary artery bypass. The patient's thorax is opened, and a substitute conduit for supplying blood to the heart is provided by engrafting a substitute vessel between a point upstream from the occlusion, such as the aorta, and a point in the coronary artery downstream from the occlusion. Coronary bypass surgery is an involved and delicate procedure, entailing significant risk and expense to the patient. Many patients are unable to benefit from bypass surgery.
In an alternative approach to relieving an occlusion of an artery, drugs are administered to cause the vessels to dilate. Not all patients can use such drugs, however, and the results are generally only temporary, as the occluding process can continue, eventually blocking even the dilated vessel.
In still other approaches, generally termed percutaneous translumenal angioplasty, an instrument for dilating the occluded artery is introduced, generally by means of a catheter, through an opening in the skin and through an opening in the wall of a large artery such as the brachial artery or the femoral artery, and passed within the arterial lumens to the site of the occlusion. In balloon angioplasty, for example, a fine guide wire is first passed to the site of the occlusion through the lumens of major arteries, observed by radiography as it progresses; then a catheter having a balloon near its tip is passed over the wire to the site, also within the arterial lumens; and finally the balloon is inflated at the site of the occlusion to stretch the walls of the artery and open the lumen. The results of balloon angioplasty can also be temporary, as the occluding process in 30-40% of patients can continue at the site until the vessel is again blocked. Moreover, the procedure carries risks of perforation or acute occlusion of the arteries by the instrument, and the flow of blood through the vessel being treated is interrupted for a time during the procedure. Only selected patients can benefit from balloon angioplasty, leaving many patients with no viable treatment, including patients having atheromas involving long segments of vessels, or having diffuse distal artery disease, or having arteries too tortuous to permit passage of guidewires.
In a variety of industrial and medical applications, useful results can be obtained by directing laser energy at a site. For example, various materials melt or vaporize upon absorption of laser energy, and parts constructed of such materials can in effect be cut or welded to achieve a desired result. Laser energy can be used in surgery for alteration or removal of tissues or obstructions or deposits by directing the energy at the matter to be altered or removed.
In a surgical technique known as laser angioplasty, conventional light guides using fiber optics have been employed for directing laser energy onto arterial plaque formations to ablate the plaque and remove the occlusion. Individual optically conducting fibers are typically made of fused silica or quartz, and are generally fairly inflexible unless they are very thin. A thin fiber flexible enough to pass through a course having curves of small radius, such as through arterial lumens from the femoral or the brachial artery to a coronary artery, typically projects a beam of laser energy of very small effective diameter, capable of producing only a very small opening in the occlusion; moreover the energy is attenuated over relatively small distances as it passes within a thin fiber. Small diameter fibers can tend to mechanically perforate vessels when directed against the vessel wall as they are passed within the vessel toward the site.
In order to bring a sufficient quantity of energy from the laser to the plaque, light guides proposed for use in laser angioplasty usually include a number of very thin fibers, each typically about 100 to 200 microns in diameter, bundled together or bound in a tubular matrix about a central lumen, forming a catheter. Laser energy emerging, from a small number of fibers bundled together in known such catheters produces lumens of suboptimal diameter which can require subsequent enlargement by, for example, balloon dilation. Such devices do not remove an adequate quantity of matter from the lesion, and their uses are generally limited to providing access for subsequent conventional balloon angioplasty.
Moreover, although individual fibers of such small dimensions are flexible enough to negotiate curves of fairly small radius, a bundle of even a few such fibers is much less flexible, and use of laser angioplasty has as a practical matter been limited to the larger, straighter blood vessels such as, for example, the large arteries of the leg, in which the laser energy is conducted by the light guide over only relatively short distances on a relatively straight course. Coupling mechanisms for directing laser energy from the source into the individual fibers in a light guide made up of multiple small fibers can be complex, including lenses and mechanisms by which the individual fibers can be addressed serially by the source beam. Improper launch of the laser energy into such a light guide can destroy the fibers, ruining the instrument and endangering the patient.
More flexible light guides can be provided by filling a flexible tube with a liquid material whose refractive index is less than that of the tube wall material. H. F. Eastgate, U.S. Pat. No. 4,045,119, describes a liquid core light guide, having a plug at each end of the tube to seal the liquid in, for transmitting laser energy at high power from a laser source such as a pulsed laser to an area of application.
The presence of blood near the distal end of such instruments can prevent laser light from reaching its appropriate target, such as for example arterial plaque or a blood clot. Moreover, absorption of laser energy by blood or blood components can result in generation of heat or formation of detonations, which can damage adjacent vessel walls.